Rock drilling tool

ABSTRACT

The present invention relates to a rock drilling element for percussive drilling that is designed to reduce the stress usually developed on a thread joint when the shock wave transmits from a slender portion to a thicker part. The rock drilling element has an elongated body comprising a first portion and a second portion. The first portion has a female or male thread intended to be connected to a drill rod or a drill tube. The first portion has an outer diameter approximately equal to the major diameter of said thread. The second portion has a male or female thread intended to be connected to a drill bit or a guide tube. The second portion forms a guide portion for radial guiding in a hole being drilled. The length of the first portion is at least 500 mm, such that the thread joint is moved away from an unfavorable reflection area. Furthermore, the present invention relates to a drill string and a method of transferring impact energy in a drill string.

BACKGROUND OF THE INVENTION

The present invention relates to a rock drilling element for percussiverock drilling, a drill string and a method of transferring impact energyin a drill string according to the preambles of the independent claims.

Drilling straight holes to locate the explosives at the right place withgood spacing and burden has always been considered as a must in theindustry of drilling. It is quite easy to drill a straight hole with adown-the-hole machine whose percussion piston is located immediately ontop of the drill bit. It is more difficult to ensure a straight holewith a top hammer machine where the percussion piston hits a drillstring, including from 1 to 10 rods.

When there are particularly high safety demands on blasting, owing tothe proximity of buildings, utility services or other installations, itis crucial for the blast holes to be drilled with the greatest possibleprecision. Indeed, high drilling precision is one of the mostfundamental ingredients of a safe and accurate blasting result.

Drill rods are somewhat flexible, which basic specification allows themto drill through difficult rock at an acceptable deviation, but alsowith reasonable low fatigue stresses in the body and the connectingends. This feature allows the drill rods to achieve a good service lifeand grants to the top hammer drilling a low cost per drilled meter.

The difficulty starts when the commonly achieved deviations are nolonger tolerated. A better location of the explosives inside the rockbody becomes compulsory. A better location of the explosives will alsoallow decrease of the amount of explosives per ton of blasted rock andcan lead to substantial savings in explosives and in secondary breaking.

Many means to improve the hole straightness with top hammer drillinghave been developed over the years. The two most common means are: guidebits and guide tubes. Guide bits are provided with up to 6 or 8 splineson the external part of the skirt. The splines are the means to improvethe guiding inside the drilled hole, but they inevitably wear out, byfar earlier than the carbide buttons crushing the rock into cuttings.After some while, the drill bit is still able to drill, but the guidingmeans have vanished. Guide tubes, such as disclosed in U.S. Pat. No.6,681,875, have external diameters close to the drill bit diameter. Thevery high rigidity of the guide tube tends to keep the drill bitstraight in line. Unfortunately, the penetration rate slows down by 10to 20%. In addition, a guide tube does not withstand the percussionpower over a long period of time and inevitably breaks at the connectionto the drill bit, or forces the upper rod connected to it to break. Theresulting drilling costs are usually considered as excessive, and guidetubes are not well accepted in the field.

Other common guide systems have short splines that wear out quickly, andthe expected improvement in guiding the drill bit becomes very quicklyineffective.

OBJECTS OF THE INVENTION

One object of the present invention is to provide a drill string havingan efficient and long lasting guide means.

Another object of the present invention is to provide a rock drillingelement that avoids over-stressing of the thread joint connecting to thedrill string.

Still another object of the present invention is to provide a rockdrilling element designed with a relatively low linear weight for a highefficiency of the shock wave transmission.

These and other objects have been achieved by a rock drilling element, adrill string and a method of transferring impact energy in a drillstring such as defined in the subsequent claims with reference to thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A schematically shows a prior art drill string during transmissionof a shock wave.

FIGS. 1B and 1C schematically show two prior art drill string equipmentsduring transmission of a shock wave.

FIG. 2A schematically shows a drill string according to the presentinvention in an exploded, cross-sectional view comprising an extensionrod, a guide rod and a drill bit.

FIG. 2B schematically shows the guide rod according to the presentinvention shown in FIG. 2A.

FIG. 3 schematically shows a drill string comprising a guide rodaccording to the present invention during transmission of a shock wave.

FIG. 4A schematically shows an alternative drill string according to thepresent invention in an exploded, cross-sectional view having anextension rod, an alternative guide rod and a drill bit.

FIG. 4B schematically shows the guide rod according to the presentinvention shown in FIG. 4A.

FIG. 5 schematically shows the alternative drill string duringtransmission of a shock wave.

DETAILED DESCRIPTION OF THE INVENTION

The basic idea for the guide rod or rock drilling element 10 accordingto the present invention is to avoid at best any stress concentration ata thread joint, which inevitably occurs with the heavy guide tubes asexperienced in prior art solutions such as U.S. Pat. No. 6,681,875. Thestress concentration in a conventional guide tube is depicted in FIG.1A, and inevitably leads to an early breakage inside the thread joint 5between drill rod 1 and the guide tube 2. The early breakage has threespecific origins, namely compressive shock waves, torsional waves andstatic bending stresses.

Regarding compressive shock waves: The heavier linear mass of the guidetube 2, at least 105-250% of the linear mass of the drill string itself,reflects part of the incident shock wave 4A energy back to the drillstring and to the drilling machine, as a result of the so-called anvileffect. Therefore, the first rod 1 connected to the guide tube 2, andmore specifically its male threaded spigot, suffers from a localincrease of the compressive stress level. This phenomenon will bedescribed further in detail below, with reference to FIG. 1.

Regarding rotation mean torque and torsional waves: The lower inertia ofthe drill string versus the higher inertia of the tube section of theguide tube 3 is sensitive to the constant mean torque and the torsionalwaves traveling along the drill string. The drill rod 1, and morespecifically the male thread of the drill rod, develops high torsionalstresses at each impact, when carbide buttons of the drill bit impact onthe rock formation.

Regarding static bending stresses: For a given angular deviation of thehole, not shown, the bending stresses can be smoothened out on bothsides of a thread joint between two adjacent somewhat flexible rods. Thecase is fundamentally different when one part is a rigid guide tube 3.Because of the rigid guide tube, it can be assumed that the bendingstresses induced in the male spigot of the rod 1 directly connected to aguide tube can almost be doubled relative to stresses developed in arod-to-rod thread joint.

All the above-mentioned stresses (percussion, rotation, bending) arecombined into a resulting stress distribution, which local excessivevalue will initiate fatigue failures and will result in full breakage.

FIGS. 1B and 1C schematically show two prior art drill string equipmentsduring transmission of a shock wave. In both figures, the piston impactson a shank adapter connected to an extension rod. As can be seen fromthese figures a shock wave is i.a. dependent on the shape and length Lof the piston. In FIG. 1B the shock wave has an irregular shape withhigh maximum amplitude peak A. In FIG. 1C the shock wave has arectangular shape with constant amplitude A. The length 2L of each shockwave is however always two times the length L of the piston.

FIG. 1A shows the transmission and reflection of an incident shock wave4A delivered by a drilling machine piston. The shock wave in FIG. 1A isfor illustrative purposes based on a shock wave as shown in FIG. 1C. Adrill rod 1, partly shown at the left-hand side of FIG. 1A, is tightlythreaded to the female end of a guide tube 2 via the thread joint 5. Adrill bit 3 is connected to the other end of the guide tube, and thedrill bit is pressed against the rock to be drilled.

At time t=−1, the shock wave 4A, shown in its entire length, which meanstwice the length of the impact piston, travels along the rod 1 towardsthe right-hand side of FIG. 1A. The incident shock wave 4A is supposedto travel through the thread joint 5 without any reflected wave; such anassumption is only made for sake of discussion. A heavier thread jointwith partial reflection when the shock wave impacts on it, wouldslightly decrease the stress level of the shock wave transmitted furtherto the guide tube 3, and slightly increase the stress level in the malethread, but would not basically change the explanation. A heavy jointwould just enhance by a few percent the problem to be described.

At time t=0, the incident shock wave abuts on the heavier tube section2.

At time t=1, because of the heavier linear mass, the incident shock wave4A is split into a transmitted wave 4C traveling through the tube 2 anda reflected wave 4B of same length traveling back towards the drillingmachine. The more or less dense hatching reflects the stress amplitudein both transmitted 4C and reflected 4B waves.

The shock wave is defined by its stress level and the pulse length. Ahigh stress level (typically 200 MPa) is shown with a dense hatching attime t=−1 before the wave abuts on the heavier tube 2. The stress levelis slightly lower, and thus the hatching is less dense after the wavetravels into the tube section. The reflected wave 4B is depicted by avery loose hatching, inclined in another direction as a symbol of a wavetraveling towards the left-hand side in FIG. 1A. However, the reflectedshock wave 4B is additive to the incident shock wave 4A, and thereforethe stress level (shown by density of hatching) is maximum.

At time t=2, the first half (50%) of the incident shock wave 4A istransmitted and reflected.

At time t=3, the incident shock wave 4A travels to the right and thelength of the rod subjected to high stress level is shorter than before.For sake of simplicity and in order to minimize the number of figures,the transmitted wave 4C traveling to the drill bit now abuts on therock. The reactions at the drill bit 3 are very variable, depending onthe drill bit weight and rock hardness. It is assumed that the drill bitof same linear mass as the tube section is used, and that the rock to behard enough to withstand the drill bit motion and bit pressure. A secondreflected compressive shock wave will then be initiated.

At time t=4, the incident shock wave 4A is fully transmitted into thetube section, and therefore, the creation of the stress wave 4B comes toan end.

At time t=5, the reflected wave 4B is completed and travels to theleft-hand side towards the drilling machine.

The most unfavorable time for the thread joint 5 is from time about t=0to about t=4, when incident 4A and reflected 4B stress waves aresuperimposed.

FIG. 1A shows a triangle depicting position and time for the overlapbetween incident and reflected waves. As can be seen in FIG. 1A, thethread joint 5 is subject to the increased stress levels from time t=1to t=3. The time periods in this context are very short, since the shockwave travels in steel with a speed of about 5200 m/s, and a usual timeperiod for a shock wave to pass a drill steel cross-section is about onethird of a millisecond (0.33 ms). This short time corresponds to theincrease of stress shown by the triangle vertical base line in FIG. 1Afrom t=0 to t=4.

For example, if an incident shock wave is 200 MPa, and the reflectedshock wave 4B is 40 MPa (only 20%) resulting from the higher tubeimpedance, then the resulting stress level is 240 MPa. As a matter ofcomparison, the 240 MPa stress level would develop in a regularrod-to-rod thread joint not shown, at a drilling machine with a 44%higher energy per impact (E), as a result of the formula:

E=∫σ².dt

where σ is the compressive stress.

The conventional drill string cannot withstand a 44% increase in energyper impact. The thread joint between drill rod and guide tube, subjectedto a 44% higher stress, has proven to be the weak point of the drillstring. The fatigue cracks usually develop in the thread joint 5, andmore precisely in the male thread, limiting the life of the twocomponents to the range of 800 to 2500 drilled meters. It should benoted that a standard rod-to-rod thread joint is able to drill from 10000 to 20 000 drilled meters. Such rod and guide tube lives are commonlyrecorded on jobsites.

The object of the present invention is to avoid the higher stress levelthat currently occurs in any thread joint between the drill rod and theguide rod.

An embodiment of a drill string according to the present invention forpercussive rock drilling comprising a guide rod 10 according to thepresent invention is described hereinafter with reference foremost toFIGS. 2A and 2B. The guide rod 10 comprises an elongated first orslender portion 10A with a substantially cylindrical basic shape of adiameter D1 and a length L1 and a second or guide portion 10B with asubstantially cylindrical basic shape of a diameter D2 and a length L2.The guide rod further comprises a first or upper end 11 defined by apreferably welded-on sleeve or female portion 12 and a second or lowerend 13 defined by a spigot or male portion 14. The spigot 14 has asubstantially cylindrical external thread 15 and the sleeve 12 has asubstantially cylindrical internal female thread 16. The first portion10A has an outer diameter D1 approximately equal to the major diameterof the female thread 16. The female thread 16 is provided in a recess inthe sleeve having an abutment surface or bottom 18. The slender portion10A outer diameter D1 is approximately equal to the major diameter ofsaid thread 16. The length L1 can be defined as the distance from thebottom 18 to the closest position where the guide portion 10B has a fulldiameter D2. The length L1 is greater than the length of the piston usedin the drilling machine, i.e. at least 500 mm. The length L2 can bedefined as the distance between the ends of the guide portion 10B, whichends have full diameters D2. The guide portion diameter D2 is 105-250%of the slender portion 10A diameter. When it comes to thecross-sectional areas (in mm²) or linear mass (in kg/m) the guideportion 10B is maximum 250% of the slender portion 10A.

A flushing channel which is generally depicted 19 extends internally ofthe guide rod 10, through which a flush medium, usually air or water, istransferred. The through-going flush channel 19 is provided to leadflush medium to the rock drill bit 3 for percussive top hammer drilling.This channel is suitably centrally positioned in the guide rod.

The slender portion 10A and the guide portion 10B are preferablyessentially cylindrical. A first shoulder 25 and a second shoulder 26border the cylindrical part of the slender portion 10A at respectiveaxial ends thereof. The first shoulder 25 is provided in the vicinity ofthe female thread 16.

FIG. 3 shows the transmission of a shock wave similar to FIG. 1A withidentical shock wave transfer and reflection, applied to drill stringaccording to the present invention comprising the guide rod 10 accordingto the present invention. The guide rod 10 includes a sufficiently longslender rod section 10A, defined in such a way that the thread joint 5is definitely located outside of the triangle where incident 4A andreflected 4B shock waves overlap.

At time t=−1, t=0 and t=1, the thread joint 5 is subjected to theincident shock wave 4A, similar to any thread joint between two standardrods.

At time t=2, the incident shock wave 4A has already ended and the stresslevel is close to zero. This feature occurs before the reflected shockwave 4B reaches the thread joint 5 in opposite direction.

At time t=3, t=4 and t=5, a harmless reflected wave 4B travels acrossthe thread joint 5, having no noticeable Influence on the guide rod 10life.

The basic idea for the guide rod 10 according to the present inventionis to keep the end part or the part of the guide rod facing away fromthe drill bit 3 as identical as possible to the drill rod 1 connected toit, and therefore to avoid the negative influence of a 105% to 150%heavier linear mass of the conventional guide tube, enhancing locallycompressive, rotation and bending stresses. In order to avoid anyincrease of the compressive stresses resulting from the impact pulses inthe thread joint 5, which is the most sensitive portion, this slenderportion 10A of the guide rod 10 should have a length equal to orpreferably longer than the impact piston, which means the length of theslender portion 10A should be at least 500 mm. This slender portion 10Asimultaneously smoothens the torque pulses and the bending stressesbefore those are conveyed towards the thread joint 5 and conveyed intothe very sensitive male thread of the rod 1 connected to it.

The guide portion 10B of the guide rod is a tubular section acting as abearing in contact with the hole wall to improve the guidance of thedrill bit 3. The major reason for defining a tubular section instead offor example six long splines is deducted from field experience, that isguide tubes are considered to be less aggressive in overburden drillingand in soft rock drilling, while six splines may deteriorate the walland drive the hole to collapse. This second portion 10B is preferablycarburized or heat-treated, to withstand high wear because of heavyfriction against abrasive rock, to a surface hardness between 48 HRC and62 HRC. The second portion 10B may comprise external shallow splines inorder to increase the flushing area and simultaneously decrease theclearance between splines and hole wall, for an improved guidance.

The method according to the present invention for transferring impactenergy from a top hammer unit to a drill bit can be summarized asfollows. The top hammer unit has a piston that provides shock waves 4A.Each shock wave has a length 2L. The method comprises the steps of:

providing a drill string comprising one or more extension rods 1 orextension tubes, a rock drilling element 10 as defined above, and adrill bit 3 or one or more guide tubes connected to a drill bit 3,

connecting an end 11 of said rock drilling element 10, 10′ facingtowards the piston via a thread joint 5 to an extension rod 1 or anextension tube,

accelerating the piston,

impacting an end of the drill string to create the shock wave 4A,

allowing more than half of the shock wave 4A to pass the thread joint 5before any reflected wave 4B is allowed to be created, and

rotating and impacting said drill bit against a rock material for makinga hole therein.

The second portion 10B can in addition be altered in length, in order tooptimize the shock wave transmission to the drill bit and to the rock.An alternative guide rod 10′ according to the present invention is shownin FIGS. 4A, 4B and 5. Contrary to our previous description, the drillbit 3 linear mass is often not equal to the tube linear mass. The drillbit 3 is much heavier, and so is the thread joint from the secondportion 10B to the drill bit 3. Because of this observation, more energyis reflected backwards to the drilling machine and not transferred tothe rock.

FIGS. 4A and 4B schematically show an alternative drill string accordingto the present invention and an alternative guide rod 10′ according tothe present invention, respectively, wherein like numerals depict likefeatures as in the previously described embodiment. The alternativeguide rod 10′ comprises the advantages of the guide rod 10 and thus hasa slender portion 10A′ and a guide portion 10B′. The major differencefrom the guide rod 10 is that the length L2′ of the guide portion 10B′has been reduced. The length L1′ of the slender portion 10A′ is greaterthan the length of the piston used in the drilling machine, i.e. atleast 500 mm. The alternative guide rod 10′ further has a possibility toprovide some more energy to the rock through the second portion 10B′ ofthe guide rod 10′ and the drill bit 3 considered as a whole. The overalllength of the second portion 10B′ and the drill bit 3 is designed assubstantially half of the length of the piston of the drilling machine,which means that their overall length is substantially one quarter ofthe incident shock wave 4A. In such a configuration, the first half ofthe shock wave 4A creates a first level of stress in the guide portionplus bit assembly, while the second half of the shock wave furtherincreases the first level of stress to a higher value. The enhancedstress level is then able to push the carbide buttons some further intothe rock. This process can improve in fact the overall energy transferto the rock and the overall efficiency.

In light of the description related to FIGS. 4A, 4B and 5, the ideallength for the guide portion 10B′ plus bit 3 is theoreticallysubstantially equal to half of the piston length. In fact, optimizationby finite element analysis shows that the overall guide portion plus bitlength should be approximately one third of the piston length. Thisvalue is only an indication considering the finite element analysisbeing the only way to optimize the shock wave transfer to the rock whileconsidering the true mass distribution along the tube and bit.

In a computer simulation test, the efficiency of shock wave transmissionhas improved from 0.7245 (with a conventional full length guide tube) to0.7677 (with optimized guide portion 10B′ length), which is almost a 6%improvement of energy transfer.

It should be noted that optimizing the shock wave transmission is notcompulsory. A somewhat or even a much longer guide portion for improvedguiding inside the hole (but not optimized with regard to energytransfer) can be designed to solve different drilling situations. Forexample, when the straightness of the hole is of a higher priority thanthe penetration rate. Such a guide rod would still take advantage of thelower compression stresses, more even rotation stresses and more evenbending stresses in the thread joint 5 that will highly benefit thedrill string life.

Such a guide rod 10 and 10′ is designed to accept conventional drillbits with a skirt and a female thread. The drill bit 3 can have either astandard skirt or a guide skirt. The above-mentioned embodiments of aguide rod according to the present invention preferably has a peripheralcontact (also called shoulder contact) between the guide rod 10, 10′ andthe drill bit 3. The major reason for having the shoulder contact arounda large thread is to provide the shock energy precisely where it isgoing to be useful at the peripheral buttons of the drill bit.

The drill bit 3 can alternatively be designed with a male threadedspigot to be inserted inside the guide rod having a corresponding femalethread. A carburized guide portion able to withstand high wear can inthis context be the sole means for guiding inside the hole, such thatthe drill bit 3 needs no integral guiding devices.

The guide rod has been shown up to now with a bit directly connected toit. It is also possible to use the guide rod as an intermediate elementconnecting together two drill string sections of differentcross-sectional area (in mm²) or linear mass (in kg/m). For example, a60 mm drill rod delivers the impact pulses to the guide rod, which inturn is connected to one or more guide tubes with heavier linear mass.The drill bit 3 is finally connected to the last guide tube.

The drill string of rods could alternatively be a string of drill tubeswherein the guide rod 10, 10′ then is replaced by a guide tube ofsimilar geometry but with greater dimensions. Such a guide tube shouldbe essentially identical to the drill tubes in its upper end, and have alarger and heavier tube at its lower end to suit the drill bit diameter.

The present invention proposes a guide rod where the thread joint ismoved away from the unfavorable reflection area. Thereby, severaladvantages are obtained, namely an efficient and long lasting guidemeans that avoids over-stressing of the thread joint connecting to thedrill string and a high efficiency of the shock wave transmission.

Although the present invention has been described in connection withpreferred embodiments thereof, it will be appreciated by those skilledin the art that additions, deletions, modifications, and substitutionsnot specifically described may be made without departing from the scopeof the invention as defined in the appended claims.

1. A rock drilling element for use in percussive top hammer drillinghaving an elongated body comprising a first portion and a secondportion, said first portion having a female or male thread intended tobe connected to a drill rod or a drill tube, wherein the first portionhas an outer diameter approximately equal to the major diameter of saidthread, wherein the second portion has a male or female thread intendedto be connected to a drill bit or a guide tube, said second portionforming a guide portion for radial guiding in a hole being drilled, andwherein the length of the first portion is at least 500 mm.
 2. The rockdrilling element according to claim 1, wherein the outer diameter of thefirst portion is 90-110% of the major diameter of the thread that isassociated with said first portion.
 3. The rock drilling elementaccording to claim 1, wherein the outer diameter of the second portionis 105-250% of the outer diameter of the first portion.
 4. The rockdrilling element according to claim 1, wherein the cross sectional areaof the second portion is maximum 250% of the cross sectional area of thefirst portion.
 5. The rock drilling element according to claim 1,wherein the second portion has external splines.
 6. The rock drillingelement according to claim 1, wherein the first and/or the secondportion consist of multiple components friction welded to each other. 7.A drill string for percussive rock drilling comprising a drill bit, oneor more extension rods or extension tubes, wherein the drill stringfurther comprises a rock drilling element as defined in claim
 1. 8. Amethod for transferring impact energy from a top hammer unit to a drillbit, which unit has a piston that provides shock waves, each shock wavehaving a length, the method comprising the steps of: providing a drillstring comprising one or more extension rods or extension tubes, a rockdrilling element as defined in claim 1, and a drill bit or one or moreguide tubes connected to a drill bit, connecting an end of said rockdrilling element facing towards the piston via a thread joint to anextension rod or an extension tube, accelerating the piston, impactingan end of the drill string to create the shock wave, allowing more thanhalf of the shock wave to pass the thread joint before any reflectedwave is allowed to be created, and rotating and impacting said drill bitagainst a rock material for making a hole therein.